We report the identification of LP 400-22 (WD 2234+222) as a very low-mass and high-velocity white dwarf. The ultraviolet GALEX and optical photometric colors and a spectral line analysis of LP 400-22 show this star to have an effective temperature of 1108

2Kawkaet

al.

Fig.1.—TheenergydistributionofLP400-22combiningallavailable11000Kdataandlogcomparedg=6.50to(seeour§3).

H-richmodelspectrumatTe =TABLE1Photometry

BandLP400-22LP400-21FUVa18·NUVa18.18.38.18±19±0.09mag±00..0805magmag······BV171818R17.338.14±±0.04mag17.742±··0···

.025magmagI

17.219±00..025021magmag17..202210±±00..023024magmag

15.14.177.933±340±0±0.0.021.023023

magmag

aThemeanofthesevaluesareusedinthispaper.

Table1presentstheopticalandultravioletphotometryandFigure1showstheenergydistributioncomparedtoasyntheticspectrum.

2.2.Spectroscopy

Weobtainedalow-resolutionspectrumofLP400-22usingtheR-Cspectrographattachedtothe4mtele-scopeatKittPeakNationalObservatory(KPNO)on1988October6.TheBL250grating(158lines/mm)wasusedtoobtainaspectralrangeof3500to6200 Adispersionof5.52 witha

Aperpixelandaresolutionof14 A.

LP400-22wasre-observedusingtheDualImagingSpectrogram(DIS)attachedtothe3.5mtelescopeattheApachePointObservatory(APO)on2001July10andOctober14.The1200lines/mmgratingwasusedtoobtainaspectralrangeof3800to4600 Awithadisper-sionof1.6 Aperpixel,andthe830.8lines/mmgratingwasusedtoobtainaspectralrangeof6180to7210 withadispersionof1.3 Aperpixel.A1′′

A

.5slitwasusedtoobtainaspectralresolutionof～2 Aintheblueand～2.6 Ainthered.

3.DETERMININGTHEPARAMETERS

InouranalysisofLP400-22,weusedagridofcom-putedpurehydrogenLTEplaneparallelmodels(see

Kawka&Vennes(2006)andreferencesthereinforde-tails).ThegridofmodelsextendfromT16000K(instepsof1000K),from18000e =7000toto32000K(instepsof2000K)andfrom36000to84000K(instepsof4000K)atlogg=6.0to9.5(instepsof0.25dex).Allourloggvaluesareincgs.Wealsopreparedcorrespond-inggridsofsyntheticspectra,oneofwhichincludesthee ectofLyαsatellites(Allard&Koester1992),andtheotherexcludesthate ect.

3.1.Photometry

Usingourspectralgrid,wehavecalculatedsyntheticoptical(BVRI)andultraviolet(FUV/NUV)colors.Figure2showstheobservedphotometriccolors(V FUVversusFUV NUVandB VversusV R)ofLP400-22comparedtosyntheticwhitedwarfandmain-sequencecolors.WeusedKuruczsyntheticspectra(Kurucz1993)tocalculateourmain-sequencecolors.IntheUV-opticaldiagram(V FUV/FUV NUV)ofFigure2weshowtwosetsofWDsyntheticcolors.Thegridshowninblackincludesthee ectofLyαsatel-lites(Allard&Koester1992)ascomparedtothegridingreenwhichexcludesthem.Acomparisonofthetwogridsshowsthesigni cante ectthattheLyαsatelliteshaveontheUVcolorsatTe <paringtheUV-opticalphotometryofLP400-22tothewhitedwarfgrid,alowsurfacegravitylogg～6andane ectivetem-peratureof～11000Kisimplied.Theopticaldiagram(B V/V R)inFigure2con rmsthewhitedwarftemperatureof11000Kandthelowsurfacegravity.However,whencomparingthephotometrytomain-sequencecolors,aA3VspectraltypeisimpliedintheopticalandaB8Vspectraltypeintheultraviolet.There-fore,thedataareincompatiblewithmain-sequencecol-ors.OpticalandUVcolorsareusefultodistinguishwhitedwarfsfrommain-sequencestars.

3.2.Spectroscopy

TheBalmerlinesofLP400-22wereanalyzedinall

threeavailablespectrausingaχ2minimizationtech-nique.Thequoteduncertaintiesarestatisticalonly(1σ).TheBalmerlines(HβtoH9)intheKPNOspectrumwere ttedwithmodelspectrawhich weresmoothedtotheinstrumentalresolutionof14A,toobtainT±350Kandlogg=6.48±0.27.Forthee =11000twohigh-resolutionAPOspectrawe ttedHαandHγtoH9withmodelspectra,toobtainT.46e =11060±180Kand11160±250K,andlogg=6±0.13and6.22±0.10.ThesyntheticspectrausedintheanalysisoftheAPOspectraweresmoothedwith agaussianpro letotheinstrumentalresolutionof2A.Notethatthedisprep-ancyinthesurfacegravitiesfromthe2APOspectraaremostlikelytheresultofuncertaintiesinthe uxcal-ibrationaroundthehigherBalmerlines.TheBalmerline toftheKPNOspectrumisshowninFigure3.Thesemeasurementsclearlycon rmthatLP400-22isawhitedwarfwithalowsurfacegravity.ThecalculatedweightedaverageofthetemperatureandsurfacegravityisTe =11080±140Kandlogg=6.32±0.08.

Weusedtheevolutionarytracksforhelium-corewhitedwarfsofAlthausetal.(2001)andSerenellietal.(2001)todetermineamassof0.17±0.01M⊙andacoolingageof5±1×108years.Notethatthecoolingageofthewhitedwarfissensitivetothemassofthehydrogen